1. Field of the Invention
The present invention relates to an operational amplifier, and more particularly to an operational amplifier that is composed of MOS transistors and capable of obtaining a relatively large output current with a low voltage.
2. Description of the Related Art
FIG. 2 is a structural diagram showing an example of a conventional operational amplifier.
The operational amplifier is composed of a differential input section 10 that amplifies a differential voltage between two input signals inputted to an inverting or inverse input terminal 1 and to a non-inverting or uninverse input terminal 2, an amplifying section 20Z that amplifies an output signal from the differential input section 10, an output section 30Z that outputs a signal amplified by the amplifying section 20Z to an output terminal 3 with a low output impedance, and a bias generating section 40 that generates a bias voltage necessary for the respective sections.
The differential input section 10 includes a p-channel MOS transistor 11, and a source thereof is connected to a supply voltage VDD. A bias voltage VB1 is applied to a gate of the p-channel MOS transistor 11 from the bias generating section 40. A drain of the p-channel MOS transistor 11 is connected with sources of p-channel MOS transistors 12 and 13, respectively, and gates of those p-channel MOS transistors 12 and 13 are connected to the inverse input terminal 1 and the uninverse input terminal 2, respectively. The drain of the p-channel MOS transistor 12 is connected to a drain and a gate of an n-channel MOS transistor 14 as well as a gate of an n-channel MOS transistor 15, respectively. Sources of the n-channel MOS transistors 14 and 15 are grounded to a ground voltage GND. Drains of the p-channel MOS transistor 13 and the n-channel MOS transistor 15 are connected to a node N1, respectively, and a signal V1 is outputted to the node N1 from the differential input section 10.
The amplifying section 20Z includes a p-channel MOS transistor 26, and the supply voltage VDD is applied to a source of the p-channel MOS transistor 26. The bias voltage VB1 is applied to a gate of the p-channel MOS transistor 26 from the bias generating section 40. A drain of the p-channel MOS transistor 26 is connected to a node N2 which is connected with sources of an n-channel MOS transistor 27 and a p-channel MOS transistor 28, respectively. Bias voltages VB2 and VB3 are applied to gates of the n-channel MOS transistor 27 and the p-channel MOS transistor 28 from the bias generating section 40, respectively. Drains of the n-channel MOS transistor 27 and the p-channel MOS transistor 28 are connected to the node N3, respectively, and the node N3 is connected with a drain of an n-channel MOS transistor 29. A gate of the n-channel MOS transistor 29 is connected to the node N1, and a source of the n-channel MOS transistor 29 is grounded to the ground voltage GND.
The output section 30Z is composed of a p-channel MOS transistor 38 and an n-channel MOS transistor 39, and a source, a gate and a drain of the p-channel MOS transistor 38 are connected to the supply voltage VDD, the node N2 and the output terminal 3, respectively. A drain, a gate and a source of the n-channel MOS transistor 39 are connected to the output terminal 3, the node N3 and the ground voltage GND, respectively.
In the operational amplifier thus structured, the differential voltage between an input signal VI1 supplied to the inverse input terminal 1 and an input signal VI2 supplied to the uninverse input terminal 2 is amplified by the differential input section 10 and then outputted to the node N1 as the signal V1. The signal V1 is amplified by the amplifying section 20Z and then supplied to the gate of the n-channel MOS transistor 39 in the output section 30Z from the node N3. Also, a signal that permits a given output current to flow in the output section 30Z is supplied to the gate of the PMOS 38 in the output section 30Z.
With the above structure, the differential voltage between the input signals VI1 and VI2 is amplified and an output voltage VO is outputted from the output terminal 3.
However, the conventional operational amplifier thus structured by the MOS transistors suffers from problems stated below.
FIG. 3 is a graph showing an example of the characteristic of the MOS transistor.
In FIG. 3, assuming that the supply voltage VDD is 2 V, a relation of a voltage Vgs between the gate and the source of the n-channel MOS transistor 39 in the output section 30Z and a drain current Id thereof is represented with the gate width W of the MOS transistor as a parameter. In this example, the gate length L is set to 1 μm.
As shown in FIG. 3, if the voltage Vgs between the gate and the source of the n-channel MOS transistor 39 is held constant, it is necessary to widen the gate width W in order to obtain a large drain current Id. Also, it is apparent from the graph that the larger the voltage Vgs between the gate and the source is, the narrower the gate width W necessary for obtaining a given drain current Id.
In order that the operational amplifier structured as shown in FIG. 2 is operated with a low supply voltage VDD such as 3 V to obtain a large output current such as 200 mA, each of the gate widths W of the p-channel MOS transistor 38 and the n-channel MOS transistor 39 in the output section 30Z is required to be set to about 3 mm. For that reason, the size of the MOS transistors 38 and 39 in the output section 30Z becomes extremely large, resulting in such a problem that a pattern area serving as the integrated circuit increases.